A History of Mathematics From Mesopotamia to Modernity

242 A History ofMathematics

Let us look at an example. To define the sine and cosine functions in chapter VIII (general

topology), Section 2 (‘measure of angles’),^6 Bourbaki used the group isomorphism from the (mul-

tiplicative) unit circleUin the complex numbersC=R^2 to the quotient groupR/Z. He then

‘endowed’ the setAof ‘half-line angles’(̂ 1 , 2 )(see Fig. 1) with a group structure, and showed it

was isomorphic toU. Finally:

1. if θis an angle inA, you define cosθ to be the real part of the complex number inU
   corresponding toθ;


2. ifxis a real number, you define cosxusing some homomorphism fromRtoA.

At this point, Bourbaki points out the need to decide the least positive value ofx∈Rwhich

corresponds to 1∈U, and discusses the merits of 360, 400, and a number called 2π(‘we will prove

the existence of such a number later’). [This account is of course simplified; in Bourbaki it takes

four pages, with everything proved.] As a reward, you finally get one of the author’s rare pictures—

the graphs of the trigonometric functions (Fig. 2).

This hardly qualified as a philosophy in the sense that the great systems of Russell, Brouwer,

and Hilbert did, but it was certainly a practical ideology, and defined an orthodoxy about what

one liked or disliked in mathematics. Much has been written, particularly by opponents, about

the hegemony of Bourbakist ideas in France from 1945 on, and their insistence that their way

D 2

D 1

u

Fig. 1The ‘half-line angle’(̂ 1 , 2 )is the angle of rotation from the first line to the second.

y

y

y= sinax

y= cosax

xx

1

–1

0

a

• 4
  a


• 40
  a
  4

a

2

a

4

3 a

4

3 a

4

a

2

Fig. 2The graphs of sinx, cosx, and tanxfrom Bourbaki’sTopologie GénéraleVIII, p. 105.

6. Which, as we have mentioned, Landau did using series.